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PULSED NEUTRON-NEUTRON BASIC PRINCIPLE:
The formation is subjected to a pulse of high-energy neutrons (14MeV) from a neutron generator. These pulses are repeated at a certain repetition rate of 20 pulses per second. The thermal neutron population is sampled between pulses and its rate of decay can be computed.
NEUTRON INTERACTIONS:
Neutrons may interact with matter in a variety of modes. Because it is rare that neutrons are absorbed until they lose most of their energy, each neutron often has many interactions in the process of losing its energy. This process is also called slowing down. The characteristics of some of these interactions can be used to predict the formation properties. In well logging, there are four major types of interactions between a neutron and a target nucleus that can be used for formation evaluation: inelastic scattering, elastic scattering, absorption or capture of fast and slow or thermal neutrons. The type of interaction that is most probable to happen is function of neutron energy.
LIFE OF A NEUTRON:
The path a neutron takes as it scatter, is the change in direction at a certain distance, sometimes even toward the source is erratic, but on average, it gradually moves away from the source. With each interaction, the neutron loses some of its kinetic energy. This
continues, until it has a value just above thermal energy (0.025 eV). Up to this point the velocity of the neutron was so much higher than the target formation nuclei, that the target nuclei could be treated as being stationary. This is no longer the case, now that the neutron's energy is just above thermal energy. The energy of the thermal neutron is about equal to the
thermal vibration energy of the formation nuclei. Subsequent collisions will, on the average, maintain the neutron's energy in equilibrium with the thermal energy of the formation nuclei.
HYDROGEN INDEX
The neutron energy loss for any particular collision depends upon the mass of the neutron and the mass of the element or particle being struck, which is demonstrated in picture below.
In the first instance, the particle being struck has a larger mass than the neutron. A small amount of energy is transferred to the particle, but the neutron bounces back, retaining the majority of its kinetic energy. In the second case, the neutron basically runs over the particle, transferring some energy to it and continues with most of its energy. The greatest energy loss results, when a neutron
collides with a particle or atom of an equal mass. This is shown in the third instance. Here, all or nearly all of the neutron's kinetic energy is passed to the equally massed particle, which is similar to what happens when two equally massed billiard balls collide head-on.
